In an application field of narrowband video communication, the bit-rate for video image encoding needs to be reduced due to a limitation of the transmission bandwidth. With a lower bit-rate, the transmitted image is most probably in an over-compression state, and effects (such as blocking effect and quantization noise) resulted from this, will bring damage to the subjective quality of the video image obviously. In a encoder, over-compression mainly results from over-quantization, and there are two specific cases: (1) a too high quantization coefficient Qp brings a too large quantization step, thereby detail changes of the video image can not be reflected, thus a high frequency component of the video image is badly distorted, and detail loss of the video image is serious. (2) Macroblock boundary effect, as the macroblocks at two side edges of a video image frame may have different coding modes, and the each macroblock may select different quantization coefficients, which result in a break of boundary energy of the video image, that is, the boundaries of the compressed and encoded adjacent image blocks are discontinuous, which resulting in a obvious blocking effect. In the conventional international standards for video image compression and encoding techniques, for example H261/H263/H264, MPEG4 and so on, the image information in spatial domain is generally converted into frequency domain based on discrete cosine transform (DCT) method, and then some of the converted DCT coefficients are quantized and coded. The blocking effect generated in the video images communication compressed in a low bit-rate by a traditional encoding and compression method is one of the important factors which results in image distortion. The blocking effect seriously affects the subjective quality and objective quality (PSNR) of video image communication.
It is proved by experiments that, for a given input image, there is a critical point when decreasing the allocated bit-rate during the encoding process. When the allocated bit-rate is below the critical point, the reconstructed image cannot reserve enough image texture information in an original temporal resolution and spatial resolution. The present invention provides a new method for video image processing, that is, an image processing method for Rate Distortion Optimization (RDO) based Adaptive spatial-temporal Resolution Frame (AstRF), wherein, when the encoder detects that the allocated bit-rate is below the critical point, appropriated temporal resolution and spatial resolution in an allocated specific bit-rate are found automatically based on the principle of rate distortion optimization, then after the decoding of the decoder, the resolution of the input image is recovered according to a certain algorithm. The image processing method in accordance with embodiments of the present invention may obviously reduce damage to the subjective quality of the video image, which is resulted from over-compression of the transmitted video image in low bit-rate.